[media] DocBook: add dvb_ca_en50221.h to documentation

There are already some tags at dvb_ca_en50221.h, but using a
different format. Convert them, fix a few entries and add
to the device-drivers DocBook.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>